metrics: Prometheus text exposition for /metrics

/metrics was Prometheus-ish name/value text with a custom banner and
text/plain Content-Type, so strict scrapers could fail.

Emit Prometheus text 0.0.4 (# HELP, # TYPE, counter|gauge) with stable
metric names, and set Content-Type to text/plain; version=0.0.4; charset=utf-8.

steamcache/negative_cache_test.go

@@ -61,6 +61,9 @@ func TestNegativeCache404(t *testing.T) {
 	if !bytes.Contains(mrec.Body.Bytes(), []byte("negative_cache_hits")) {
 		t.Errorf("/metrics missing negative_cache_hits:\n%s", mrec.Body.String())
 	}
+	if ct := mrec.Header().Get("Content-Type"); ct != "text/plain; version=0.0.4; charset=utf-8" {
+		t.Errorf("/metrics Content-Type=%q, want Prometheus text 0.0.4", ct)
+	}
 }
 
 func TestNegativeCache410(t *testing.T) {
@@ -207,4 +210,13 @@ func TestWriteTextNegativeCacheHits(t *testing.T) {
 	if !bytes.Contains(out, []byte("negative_cache_hits 0\n")) {
 		t.Errorf("/metrics missing negative_cache_hits 0:\n%s", out)
 	}
+	if ct := resp.Header.Get("Content-Type"); ct != "text/plain; version=0.0.4; charset=utf-8" {
+		t.Errorf("/metrics Content-Type=%q, want Prometheus text 0.0.4", ct)
+	}
+	if !bytes.Contains(out, []byte("# HELP negative_cache_hits ")) {
+		t.Errorf("/metrics missing # HELP negative_cache_hits:\n%s", out)
+	}
+	if !bytes.Contains(out, []byte("# TYPE negative_cache_hits counter")) {
+		t.Errorf("/metrics missing # TYPE negative_cache_hits counter:\n%s", out)
+	}
 }